Diagnostic sensor

ABSTRACT

A sensor of force or viscosity or other attributes of a fluid comprises a mechanical resonator ( 10 ) including an element ( 11; 18; 123 ) of which the stiffness at least partially determines a modal shape of the resonance of the resonator and means ( 21-23 ) for measuring a variation of a measure of the resonance as the stiffness of said element changes. The resonator ( 10 ) may comprise two beams ( 10   a,    10   b;    120, 121 ) connected at or near one end by a yoke ( 12; 122 ) which provides a clamped condition of the resonator at said one end and connected at or near another end by said element.

This invention is concerned with the use of a vibrating resonantstructure for the measurement of physical and chemical properties offluids and solids.

It is known to measure an attribute such as the density or viscosity ofa medium, whether solid or fluid, by measuring some characteristic orparameter of vibration of a vibratile structure. Some examples whichillustrate the general state of the art are shown in U.S. Pat. Nos.5,023,560, 5,363,691 and 5,670,709.

The present invention is based on measuring vibration of amplitude,phase or frequency of a mechanical resonator as a result of a change inmodal shape of a resonant system, caused for example by a change instiffness of a beam element which form part of or is coupled to theresonator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate beam resonators various resonant systems.

FIGS. 3 and 4 illustrate various resonant characteristics.

FIGS. 5 to 8 illustrate various beam resonators.

FIG. 9 illustrates a particular embodiment of the invention (FIG. 9A)and characteristics of its resonant modes (FIG. 9B).

FIG. 10 illustrates a resonant characteristic.

FIG. 11 illustrates another embodiment of the invention.

FIG. 12 illustrates another embodiment of the invention.

FIG. 13 illustrates the embodiment shown in FIG. 12 but vibrating in adifferent mode.

FIGS. 14 a and 14 b illustrate the change of a resonator from a‘clamped-pinned’ state to a ‘clamped-free’ state.

DETAILED DESCRIPTION

Various embodiments are described by way of example to illustrate theprinciples employed in the invention.

FIG. 1 illustrates a beam resonator 10 comprising two substantiallyparallel beams 10 a and 10 b which are rigidly connected at theirextremities. The connections of the beams at their extremities aresubstantial and yokes 11 and 12 thus formed at the extremities have ahigh stiffness. The beams are therefore referred to, in the terminologyof modal beam analysis, as ‘clamped’. The vibration of the beams isdenoted by the arrows Y.

Such a resonator, referred to herein as ‘clamp-clamp’, will resonate ata first mode natural frequency given by the equation:f=22.3733√{square root over ( )}(EI/ML ⁴)  (1)

-   -   f=Frequency    -   E=Young Modulus    -   I=Mass Moment of Inertia    -   M=Mass/Unit Length    -   L=Beam Length

If the connecting yoke 11 at one end of the resonator 10 issubstantially weakened the beams can no longer be considered clamped, asthey are allowed a degree of rotation about their connection point, inthe terminology of modal beam analysis the yoke is then approaching the‘pinned’ condition. This condition is shown in FIG. 2, and the‘clamp-pinned’ system will now resonate at a natural frequency given bythe equation:

 f=15.4182√{square root over ( )}(EI/ML ⁴)  (2)

From equations (1) and (2) it can be seen that the transition from fullyclamped to fully pinned results in an approximately 30% change infrequency. If the yoke connection is lost the beams are referred to as‘free’, and the clamped-free system now resonates at a natural frequencygiven by the equation:f=3.516√{square root over ( )}(EI/ML ⁴)  (3)

From equations (2) and (3) it can be seen that the transition from fullypinned to fully free results in an approximately 77% change infrequency.

In addition to a change in frequency, the modal shape of the clamp-clampsystem differs significantly to that of the clamp-pinned arrangement. Ageneralised view of the displacement of the beam in clamped and pinnedmode is shown in FIG. 3. The curve 30 illustrates the variation inmaximum displacement of the beam over its length when the yokes 11 and12 are ‘clamped’. The curve 31 illustrates the correspondingdisplacement when the yoke 11 is ‘pinned’. FIG. 4 shows the variation oflocal beam bending with respect to position along the beam for theclamp-clamp state (curve 40) and the clamp-pinned state (curve 41).There are further modal changes as the clamp-pinned condition progressesto clamped-free.

A sensor, such as a piezoelectric device, responding to localised beamflexure would exhibit a signal response with position along the lengthof the beam in a manner similar to the profiles shown in FIG. 4. As wellas a voltage amplitude variation with length there can also be observeda polarity change, or phase change, of the vibration signal along thelength.

In summary, the transition from a clamp-clamp state to a clamp-pinnedstate, or from a clamp-pinned state to a clamp-free state results inchanges in frequency, amplitude and phase of vibration relative toposition on beam. It also follows that a change from clamp-clamp toclamp-free will obviously have a similar result. The measurement ofchange of these parameters in response to an event influencing the modalclassification of the resonator (e.g. clamp-clamp to clamp-pinned,clamp-pinned to clamp-free) forms the basis of this invention.

In applications where there is depletion of a substance or build-up ofmaterial, such as corrosion or scaling, the actual substrate may form astiffening member on the beam yoke and thus contribute to the status ofthe yoke as clamped or pinned.

FIG. 5 shows a simple embodiment of a dual beam system, similar to thatshown in FIG. 1. Depletion of material at the connection of the beamsforming yoke 11 produces a thinner, less substantial connection as shownat 13 in FIG. 5 and can produce a change to a pinned state at this endof the resonator. However in general such a system would requiresignificant depletion of material to manifest a modal change.

Improvements on this basic system are shown in FIG. 6 and FIG. 7; inboth these cases the simple connection yoke 11 is replaced by a boxsection 15. The section achieves its stiffness from the spatiallyseparated members 14 and 14 a, and a small reduction of thickness of apart of either member 14, 14 a will manifest a substantial change inrigidity—thus altering the stiffness of the section.

FIG. 6 particularly shows a system where in the rigidity of the boxsection is modulated by the longitudinal stiffness of the member 14.FIG. 7 shows the segments 16 and 17 between the members 14 and 14 a;they will significantly influence the rigidity of section 15 if theirflexural or longitudinal stiffness is altered.

FIG. 8 shows a refinement of the system in FIG. 6 and allows for a‘bolt-on’ section stiffener 18 to be used to join the beams at the endin place of member 14. The stiffener 18 is secured by bolts 19 to eachof the beams 10 a and 10 b, This forms a convenient means of selectingthe material, shape and size of a section member to suit a particularapplication.

FIG. 9A illustrates a specific embodiment based on the system shown inFIG. 8, although in principle any of the systems previously describedcould be used. Piezoelectric transducers are strategically placed toindicate the amplitude and phase of the flexure of the beam at aspecific location. The signal from each transducer relates directly tothe modal pattern formed by the clamp or pinned condition of the yoke.Lateral vibration of the beam structure is shown by the arrows Y.

In the system shown in FIG. 9A, piezoelectric transducers 21, 22 and 23are disposed at different locations along the inner surface of the lowerbeam 10 b in order to obtain a measure, represented by the relevantpiezo output voltage, of the displacement of the beam at those locationswhen the system is in a resonant vibratory mode induced by a drivepiezoelectric transducer 24 disposed (in this example) at the clampedend of the resonant beam system. In this system the transducer 23 actsas a reference, because it is located close to a node and thedisplacement of the adjacent part of the resonator is minimal. The drivetransducer 24 may have a regenerative feedback connection (known per sein the art) from one or more of the sensing transducers 21, 22 or 23.Electromagnetic drive and sensing transducers may be used in place ofpiezoelectric transducers. Other forms of transducer, such ascapacitative, optical or acoustic transducers may be used asappropriate.

FIG. 9B is a graph of piezo voltage against distance measured along thebeam, the curves 90 and 91 being for the clamped and pinned conditionrespective at the end 11. The particular piezo voltages are given by theintersections of the projection lines 21, 22 a and 23 a (throughrespective transducers 21, 22 and 23) with the curves 90 and 91.

The embodiment shown in FIG. 9 includes an enclosure 93 for the sensor,the enclosure comprising a tube which has a gland 94 for wires to passto the transducers through a closed end of the tube. O-ring seals 95 and96 are disposed between the beam structure 10 and the tube near the endsof the tube, from the open end of which the box section 15 protrudes.

FIG. 10 illustrates the variation of the piezoelectric voltages V_(a)and V_(b), from the sensors 21 and 22 (curves 101 and 102) and thefrequency of resonance (curve 103), as a function of the decreasingstiffness of the section stiffener 18. Curve 101 exhibits a phase change(shown at 104). The curve 100 shows the substantially constant piezovoltage V_(ref) obtained from the transducer 23. It follows that theprogress of any physical, chemical, or biological effect leading to thedepletion or build-up of the sacrificial section stiffener material canbe monitored by measuring the modulation or variation of thesepiezoelectric sensor signals over time. As an example, a sectionstiffener made from iron will have its thickness, and hence itsstiffness depleted, in a corrosive environment over time and measurementof Va, Vb or frequency will indicate the rate of corrosion. It furtherfollows that selection of other materials in the electrochemical seriescan exhibit the same corrosion/deposition effects in the appropriateelectrolyte or reactive medium.

Signal processing techniques, such as the following, can be employed toenhance the result:

-   -   (a) Division of Va by Vb will result in a ratio dependent on        section stiffness but independent of amplitude of signals or        system damping.    -   (b) Division of Va or Vb by Vref will result in a ratio        dependent on section stiffness but independent of amplitude of        signals or system damping.    -   (c) Measurement of phase of Vb will form a simple method of        indicating the point at which a specific section stiffness is        reached.    -   (d) A plurality of piezoelectric sensor mounted alone the beam        can be monitored for change of phase to indicate progress of        change of section stiffness.    -   (e) If the resonator is at fill temperature equilibrium with its        environment the modal shape will indicate section stiffness        independently of temperature.    -   (f) Frequency signal has some temperature dependency so        comparison of modal shape with frequency signal will yield both        temperature and section stiffness from a single resonator.

In general terms the invention can provide a force transducer. With thesection stiffener removed as shown in FIG. 11 (which otherwise resemblesFIG. 9) the stiffness of the pinned yoke will be altered towards theclamped state by the presence of external forces 111, 112 on the yoke,either in compression or tension. These forces can be mechanical,electrical or magnetic.

The movement of the beams 10 a and 10 b will create a velocity andtherefore a shear action within a fluid. By measuring energy loss, orthe quality factor Q, of the signal the viscous shear loss can bedetermined, and thus the fluid viscosity. Similarly, the dampingcapacity of any solid connected to the yoke can be determined from the Qof the resonant signal.

The elastic properties of a viscoelastic fluid can be derived from thechange in resonant frequency due to the stiffening of the yokes a resultof the elastic modulus of the fluid.

FIG. 12 illustrates sectionally a different embodiment in which the beamstructure 10 comprises an internal cyclindrical beam 120 and an externalcylindrical beam 121. The beams are connected by a relatively thick yokemember 122 at a ‘clamped’ end and by a yoke member 123 at the other end.Reduction of the stiffness of this yoke 123 changes the connection atthis end from ‘clamped’ to ‘pinned’.

FIG. 12 includes sensing transducers 21, 22 and 23 disposed on theexternal beam and a drive transducer 24 disposed on the yoke member 122.The beam structure is enclosed by a tube 93 which has a gland 94 andintermediate O-ring seals 95 and 96.

FIG. 13 is similar to FIG. 12 but illustrates vibration of the structurein a longitudinal mode (arrow X).

As a further example, FIGS. 14 a and 14 b shows the degeneration of aclamp-pinned structure 10 shown in FIG. 14 a to the clamp-free conditionas the member 11 at one end vanishes The structure is clamped at itsother end 140.

All the resonators can be operated in harmonic modes above the naturalfrequency. There are proportional movements in modal/frequency behaviourat higher modes than a fundamental mode.

1. A sensor, comprising: a mechanical resonator comprising two beams, ayoke rigidly connecting said beams at a first location so that saidbeams are mutually clamped at said first location, and a connectingelement connecting the beams at a second location spaced from the firstlocation along the beams, said connecting element having a stiffnessthat at least partially determines a modal shape of resonant vibrationof the beams; and at least one transducer, disposed adjacent one of thebeams at a location between said first and second locations, and beingdisposed to sense a vibrational parameter that indicates a variation ofsaid modal shape.
 2. A sensor as set forth in claim 1, wherein saidconnecting element provides a clamped connection of said beams at saidsecond location, and wherein said at least one transducer senses avariation of said modal shape from a clamped-clamped shape to aclamped-pinned shape.
 3. A sensor as set forth in claim 1, wherein saidconnecting element provides a pinned connection between said beams atsaid second location, and wherein said at least one transducer senses avariation of the modal shape from a clamped-pinned condition to aclamped-free condition.
 4. A sensor, comprising: a mechanical resonatorcomprising two parallel beams; a yoke connecting said beams together ata first discrete location; a box section connecting said beams togetherat a second discrete location spaced apart from said first discretelocation, said box section including first and second spaced connectingelements which connect the beams together, said connecting elementshaving stiffnesses that at least partly determine a modal shape ofresonant vibration of the resonator; and at least one transducerdisposed between the first and second locations to measure a vibrationalparameter that indicates variation of said modal shape.
 5. A method ofsensing, comprising: deploying a sensor comprising a mechanicalresonator comprising two parallel beams and at least two connectingelements which connect the beams together at different, space apart,locations, at least one of said connecting elements having a stiffnesswhich determines a modal shape of resonant vibration of the resonator,and at least one transducer disposed to provide a vibrational parameterwhich indicates a variation of said modal shape; exposing one of saidconnecting elements to an environment which physically alters one ofsaid connecting elements so as to alter its stiffness; and monitoring achange in said modal shape.